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From Peter Firmstone <j...@zeus.net.au>
Subject Re: Towards Internet Jini Services
Date Fri, 01 Oct 2010 11:22:57 GMT
Patricia Shanahan wrote:
> Peter Firmstone wrote:
>> Zoltan Juhasz wrote:
>>> Peter,
>>>
>>> I am (has been) interested in the Internet use of Jini. There are many
>>> issues as you have noted below. More fundamental is the model of 
>>> interaction
>>> between services. Way back I was trying to talk about these issues in a
>>> London Jini Meeting perhaps without much success. There are two main 
>>> schools
>>> of thought in service interaction; API vs messaging (aka document) 
>>> based
>>> models.
>>> Messaging relies on a relatively simple send/receive interface, the 
>>> real
>>> thing is the content. Services are meant to be intelligent enough to
>>> interpret these messages and deal with them accordingly. In the 
>>> networking
>>> world (and also in the Jini community) this is referred to as the 
>>> world of
>>> protocols. My personal belief is that these only work for simple and
>>> well-defined services like ftp and the like. If you ask agent people 
>>> they
>>> will say the opposite I suppose.
>>> The API approach that Jini takes requires up-front design work and 
>>> same say
>>> these systems will inherently be brittle. I think versioning is a 
>>> problem in
>>> both approaches.
>>
>> Versioning - Service API is like an Interface, once it is widely in 
>> use, it cannot change, the implementation can, but the API cannot, 
>> since it is installed in the client.
>>
>> However we can extend service API, just like we can extend an Interface.
>>
>> Then, new clients utilise the extended Service API, while old clients 
>> continue to use the original, I've suggested the extended service API 
>> jar archive previously, but it's also possible to add any new API 
>> classes to the proxy download codebase. In the old client it will be 
>> loaded with the proxy's ClassLoader, but not utilised (unless the 
>> Service UI uses it), but the new interface won't be loaded by the 
>> proxy ClassLoader at the new client, because the new client has 
>> already loaded the extended Service API, in a parent ClassLoader, 
>> since new clients are compiled against the extended API.
>
> The mailing list lambda-dev@openjdk.java.net contains discussion of
> "defender methods", which are intended to make it possible to specify a
> default implementation for an interface method. A class that implements
> the interface without supplying an implementation for the interface
> method would use its default implementation instead of being treated as
> an error.
>
> With that in place, it would be possible to add a method to an interface
> as long as it can be given a default implementation, without resorting
> to a new extension.
>
> Does that have any effect on River direction? Do we want to aim for
> similar flexibility?
>
> Patricia
>
>

It's an interesting feature for Interfaces.

For an Interface, that takes advantage of binary compatibility, client 
classes don't object to additional methods, but implementing classes do, 
hence the default implementation for new methods.  Allowing a default 
implementation for a new interface method, sounds like multiple 
inheritance, eg extending three abstract classes, without allowing 
object global fields.  Time will tell if it's desirable.

It may be possible to use this new interface feature in Service API, 
since it is a way of providing backward compatibility and provided that 
the Service API remains backward compatible it shouldn't be a problem, 
however I'm not sure about how Reggie will handle it.

Service API, is a jar archive containing public interfaces and classes 
common to the client, service and proxy, installed at the client, which 
the service has no control over.  If the client and Service belong to 
different people or corporations....  If there was a version management 
system on the client, like OSGi, then it might be possible to ask the 
client to update it's local copy of the Service API.  But if the client 
accesses another service and compatibility is broken between versions, 
then the client must have a copy of both Service API versions and the 
client doesn't have a way to treat each as the same class, unless it 
uses a common superclass (eg Object).

 From memory Reggie does allow for different versions of Service API, 
but they are treated as separate types, this is probably important in a 
Global sense, in case name conflicts occur.

The beauty of extending Service API is that there is always a common 
superclass in the form of the original service API, for compatibility.

Service API may span several Interface and classes, such as parameters 
and return objects, clients might export objects (unregistered services) 
passed to the service as parameters.

There's much more complexity for Service API than an Interface class, 
because it includes deployment, so I used a Simile to help developers 
understand, I didn't mean it as Metaphor though.

Instead of changing a Service API incompatibly, create a new Service 
API, create another Service that implements the deprecated Service API 
but uses the new Service from within a smart proxy, until all clients 
are updated.

Due to the complexity of Versioning within Service API, I think it's 
easier if changes to Service API are limited to internal class 
implementation changes and fixes, without changing serialization 
compatibility and without adding new classes (which would cause 
deserialization problems on the client).  The minimum standard is that 
Service API should always be backward compatible for clients that you 
want to support.

It's far simpler to extend and distribute the extension classes in a 
separate jar, that can be downloaded with the proxy for clients missing 
the additional classes.

The Service implementation and proxy are free to change any way they 
like, while they honor the Service API contract.

Cheers,

Peter.

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